Aluminising mirrors at Calar Alto: top technology at shine

Ground-based telescopes have mirrors whose reflective coating, normally made up from aluminium, has to be renewed from time to time to keep them in perfect conditions. For this reason, the best observatories in the world are equipped with coating facilities to periodically re-aluminise their mirrors. Calar Alto Observatory has two of such devices: one is capable of coating mirrors up to 2.2 m diameter, and the other one accepts mirrors as large as 3.5 m. Since these facilities are expensive and require highly skilled staff to yield optimal results, many institutions prefer not having their own aluminising systems, but bringing their mirrors elsewhere to get this service. Calar Alto Observatory has been offering this possibility since its beginning. Maybe the most outstanding mirrors bearing Calar Alto aluminium are those of the largest space telescope ever launched, ESA’s Herschel mission, in orbit since 2009.

Very recently, two impressive optical pieces have been coated at Calar Alto Observatory: the primary mirrors of the innovative, international, balloon-borne solar telescope Sunrise, and the large primary mirror for a robotic optical telescope for the University of Moscow.

Back view of Sunrise main mirror, before being aluminised at Calar Alto Observatory facilities

Shiny Sunrise

Sunriseis an international project, leaded by the Max Planck Institute for Solar System Research (Germany), aimed to the study of magnetic activity on the Sun, by means of a special balloon-borne telescope. Solar magnetism is better studied in ultra-violet radiation, and this kind of invisible light is absorbed by the Earth’s atmosphere. For this reason, the Sunrise telescope performs its observations hanging from a balloon, flying at 40 km over the Earth surface and at polar latitudes, where a continuous vision of the Sun is granted in summer time.

Sunrise main mirror after receiving its aluminium coating at Calar Alto Observatory facilities

To flight with a balloon, all pieces of hardware for the Sunrise telescope have to be as lightweight as possible. Hence, the 1 m diameter main mirror of this telescope has a special, honeycomb-like structure that confers the vitro-ceramic block the rigidity needed, not adding too much mass.

The Sunrise system performed a first flight in 2009. The hardware was recovered and now it is being refurbished and improved for an imminent second flight. Coating the mirror at Calar Alto with a new aluminium layer has been a key part of this refurbishment.

From France to Russia via Calar Alto

The company Sagem Défense Sécurité (SAFRAN Group) is close to completing an automated astronomical telescope with a diameter or 2.5 m for the University of Moscow’s Sternberg Astronomical Institute. This telescope, controlled via internet, will be installed close to the Kislovodsk solar station (Caucasus). The prime contractor has decided bringing the mirrors to Calar Alto for coating. The classical, monolithic vitro-ceramic mirrors for this telescope have received a complete surface treatment consisting of a nickel and chromium substrate to improve aluminium adherence, followed by the reflecting aluminium coating and a final transparent protective layer of silicon-based gel.

Main mirror of the 2.5 m telescope being built for the University of Moscow by Sagem, before aluminisation at the facilities of Calar Alto Observatory

Calar Alto coating facility

The reflecting quality of the mirrors of Calar Alto telescopes is periodically tested. When time comes to clean and re-coat the mirrors, the procedure follows several strictly pre-specified steps. First of all, the mirror has to be removed from the telescope, a task not as easy as it may seem, given the huge weight of these vitro-ceramic blanks: around one ton for the 1.23 m telescope, 2.5 tons in the case of the 2.2 m telescope and the incredible 12 tons of the 3.5 m primary.

The mirrors are carefully cleaned and dust, spots, and the old aluminium coating are totally removed. The mirror, deprived from its reflecting capability, is then introduced into a huge vacuum chamber. The vacuum chamber at the building of the 3.5 m telescope has an inner volume of 90 cubic metres. After closing the chamber, powerful pumps create a high vacuum inside: the inner pressure is reduced down to one millionth of one thousandth of the normal atmospheric pressure for standard coating procedures (1×10-6 mbar).

Screenshot of the new digital control system for the aluminization chambers of Calar Alto, that recently has been completely renewed by the staff of the German-Spanish Astronomical Centre

Once the intended vacuum is reached, an overwhelming electrical intensity of up to 5000 ampères is driven through a set of resistances, into which small pure (99.999%) aluminium bars had been previously inserted. In less than 80 seconds, the aluminium is completely vaporised and it expands through the space inside the chamber, coating all the surfaces it finds in its way.

In the standard procedure, the aluminium layer deposed onto the mirror surface grows up to a thickness of one tenth of one thousandth of a millimetre (one tenth of the size of a bacterium). The system versatility allows to perform non-standard coatings, including nickel-chromium layers and silicon-based protective films. Also, aluminium layers of different thicknesses can be applied.

Calar Alto offers this coating service to external entities, both public and private. Mirrors, reflectors, and antennae from many different observatories and companies around the world have come to Calar Alto to get their surfaces coated according to the maximum standards of quality: from Europe to Japan, from Earth to space, many mirrors carry Calar Alto coatings whose outstanding performance is guaranteed by the many years of experience of Calar Alto staff and the extreme high quality of the facilities available at our Observatory.